1. Field of the Invention
The present invention relates to a nickel-metal hydride secondary battery of long life which can be used for backup purpose and the like.
2. Description of Related Art
With the recent spread of portable devices, alkaline storage batteries are demanded to have higher capacity. Particularly, nickel-metal hydride secondary batteries which comprise positive electrodes having an active material mainly composed of nickel hydroxide and negative electrodes comprising a hydrogen-absorbing alloy as an active material have rapidly spread as secondary batteries of high capacity and high reliability. Furthermore, lead storage batteries or nickel-cadmium secondary batteries, which have hitherto been used for backup purpose, are now being replaced with nickel-metal hydride secondary batteries as a result of improvement in performance of nickel-metal hydride secondary batteries and from the recent view for the earth environment.
Conventional nickel-metal hydride secondary batteries will be explained below. For example, a cylindrical nickel-metal hydride secondary battery has such a structure as comprising a case in which a plate group formed by spirally rolling a positive electrode and a negative electrode with a separator interposed is inserted and an electrolyte is filled and which is sealed with a sealing plate.
Positive electrodes of nickel-metal hydride secondary batteries are roughly classified into sintered type and unsintered type. The former are prepared, for example, by sintering a nickel powder to obtain a porous nickel sintered substrate having a porosity of about 80%, impregnating the resulting porous substrate with a solution of a nickel salt such as aqueous nickel nitrate solution and then dipping the substrate in an aqueous alkali solution, thereby to produce a nickel hydroxide active material in the porous nickel sintered substrate.
The latter unsintered type positive electrodes are prepared by filling a three-dimensionally continuing spongy porous substrate comprising nickel metal and having a porosity of 95% or higher with nickel hydroxide as an active material as disclosed in JP-A-50-36935, and, at present, they are widely used as positive electrodes of nickel-metal hydride secondary batteries of high capacity.
As to this unsintered type positive electrode, it has been proposed to fill the porous substrate with spherical nickel hydroxide from the point of attainment of high capacity. According to this technique, pore size of the spongy porous substrate is about 200-500 xcexcm, and the pores are filled with spherical nickel hydroxide having a particle size of several xcexcm to several ten xcexcm. In this unsintered type positive electrode, in order to improve utilization ratio of the filled nickel hydroxide, a conductive agent is used in addition to nickel hydroxide. The conductive agent electrically connects the spherical nickel hydroxide particles. Cobalt compounds such as cobalt hydroxide and cobalt monoxide, metallic cobalt, metallic nickel and others are used as the conductive agent.
Thus, it becomes possible to fill the active material at a high density and to connects the active material electrically in the unsintered type positive electrode. The capacity can be increased as compared with the sintered type positive electrode.
Furthermore, there is a demand in the market to provide high capacity secondary batteries which use unsintered type positive electrodes and are excellent in over discharge characteristics and further improved cycle characteristics. As a method for producing a positive electrode active material for high capacity nickel-metal hydride secondary batteries for meeting the above demand, JP-A-8-148145 discloses a method which comprises coating a cobalt compound on an active material of nickel hydroxide and subjecting the cobalt compound to an alkali oxidation treatment to convert to a higher order cobalt oxide. JP-A-9-73900 discloses an improvement of the above method.
According to these methods, the nickel hydroxide powder coated with the cobalt compound is sprayed with an aqueous alkali solution under fluidization or dispersion in the heated air. As a result, it has become possible to make nickel-metal hydride secondary batteries of high energy density which are improved in active material utilization and battery characteristics such as high rate discharge characteristics and overdischarge characteristics as compared with the conventional methods in which the cobalt compound is added as an external additive.
Negative electrodes of nickel-metal hydride secondary batteries comprise a plate prepared by coating a nickel-plated perforated steel sheet with an active material mainly composed of a hydrogen-absorbing alloy to which a carbon material is added to improve the surface reaction activity or yttrium oxide is added to improve oxidation resistance of the hydrogen-absorbing alloy.
The hydrogen-absorbing alloys used mainly for nickel-metal hydride secondary batteries are AB5 alloys whose composition is generally shown by LaNi5. AB5 alloys, which are put to practical use, are LaNi5 alloys in which La is replaced with Mm and Ni is replaced with Co, Mn and Al and are shown by MmNiaCobMncAld.
The A site of the AB5 alloys comprises Mm (a misch metal: a rare earth metal mainly composed of La) and the B site comprises Ni, Co, Mn and Al, and 1: a+b+c+d=1:5. Therefore, the alloys are called AB5 alloys.
Depending on the composition of metals in the B site, corrosion resistance, hydrogen absorbing and releasing reaction rate and amount of absorbed hydrogen of the alloys can be changed, and various compositions are investigated.
Separators of nickel-metal hydride secondary batteries are mainly made of a nonwoven fabric comprising fibers of polypropylene or nylon. Separators must have such actions as physically separating the positive electrodes and the negative electrodes from each other and holding the electrolyte so that an electrochemical reaction takes place at the positive electrodes and the negative electrodes.
Polypropylene and nylon are hydrophobic for aqueous solution and have no function to hold the electrolyte. Therefore, separators of nickel-metal hydride secondary batteries are made hydrophilic by a surface treatment so as to hold the electrolyte in the batteries.
When a hydrogen-absorbing alloy is used as the negative electrode material of nickel-metal hydride secondary batteries, Mn and Al dissolve into the electrolyte from the negative electrode during long-term charging and discharging or long-term storage. If Mn and Al dissolve into the electrolyte, characteristics of the battery are deteriorated.
Thus, a main object of the present invention is to provide a long-life nickel-metal hydride secondary battery usable for backup by reducing the amount of Mn and Al which dissolve into the electrolyte due to corrosion of the hydrogen-absorbing alloy, besides, passivating the dissolved Mn and Al to inhibit deterioration of the battery characteristics. Other objects of the present invention will be readily apparent from the description of the invention.
For attaining the above object, the present invention provides a nickel-metal hydride secondary battery comprising a positive electrode comprising nickel hydroxide, a negative electrode comprising a hydrogen absorbing alloy, a separator and an alkaline electrolyte, characterized in that the battery further comprises a compound silicate. xe2x80x9cCompound silicate,xe2x80x9d as used herein, refers to a salt consisting essentially of a salt formed by silicate dioxide and a compound metal oxide comprising at least two metals.
By adding the compound silicate, the battery is inhibited from deterioration in electrochemical characteristics and, as a result, the life of the battery can be prolonged.